One example of a rotary machine is a gas turbine engine. The gas turbine engine has a rotor assembly which extends axially through the engine. A stator assembly extends circumferentially about the rotor assembly. A flowpath for working medium gases extends through the engines through the rotor assembly and the stator assembly and is bounded by elements of both assemblies.
The rotor assembly typically includes a rotor disk and a plurality of rotor blades extending radially outwardly across the working medium flowpath. The stator assembly includes an outer case and arrays of stator vanes, or struts, which extend radially inwardly across the working medium flowpath. A seal assembly is provided between the rotor disk of the rotor assembly and the strut of the stator assembly to provide a boundary to the working medium flowpath.
One example of such a construction is shown in U.S. Pat. No. 5,152,666 issued to Stripinis and Walsh entitled Stator Assembly for a Rotary Machine. The seal assembly is shown adjacent reference numerals 148 and 152. The seal assembly includes a circumferentially extending seal land which faces inwardly. The seal land is attached to the stator assembly and is circumferentially continuous. A rotatable seal element extends circumferentially about the axis of rotation A.sub.r of the machine. The rotatable seal element includes a circumferentially continuous support member and a plurality of seal elements which extend radially outwardly from the support member.
The rotatable seal elements are formed of a plurality of stacked rings having sloped transition shoulders between the rings. The outermost ring is referred to as the fin ring and the supporting rings as pedestal rings. The rings become thinner in a direction perpendicular to the circumferential direction as the seal element extends outwardly. This reduces stresses in the seal element by reducing the mass of the seal element. Each rotatable seal element is circumferentially continuous. As a result, the rings are subjected to large hoop stresses as the seal element is rotated about the axis of rotation A.sub.r.
Under operative conditions, the rotatable seal element may rub against the seal land, abrading the seal element and generating heat at the region of contact. The thermal and mechanical stresses that result coupled with the very large hoop stresses that result from the high rotational speeds of the rotor assembly may cause a crack to form after the structure has been subjected to cyclic stresses. The cyclic stresses result from mechanical and thermal stresses associated with accelerating the engine from Seal Level Take Off conditions (high rotational speeds, very hot temperatures) and decelerating the engine after cruise to idle descent and shut-off of the engine. The crack grows radially inwardly and at an explosive rate of speed once it extends from the upstream side to the downstream side of the seal element. This is in part due to the action of the hoop stresses acting on the fracture point of the crack as the crack propagates inwardly.
The hoop stresses provide a certain energy which is available to cause the crack to grow. The energy is proportional to the applied hoop stress. For each material, there is a critical factor associated with cyclic fatigue which is referred to as the Critical Stress Intensity Factor (K.sub.ic). K.sub.ic represents the stress intensity at the crack tip at which the crack growth becomes unstable and rupture of the structure occurs in a short number of cycles. K.sub.ic is expressed in units of pounds per square inch and inches to the one-half power (psi in.sup.1/2).
Another important factor is K instantaneous (Ki). Ki is the actual stress intensity factor for a crack. It is related to stress, geometry factors, and crack size. Once Ki exceed K.sub.ic, the critical stress intensity factor has been exceeded and crack growth becomes unstable and rapid. The crack may grow so rapidly as to propagate itself through the underlying support member, causing fracture of the support member coupled with disintegration of the support member and damage to rotating components downstream. As a result, once a crack starts to form in a rotating knife edge, the knife edge has only a given life and must be replaced. If the crack is missed during regular inspection intervals, the crack may propagate into the underlying support structure with concomitant harmful effects.
Accordingly, scientists and engineers working under the direction of Applicant's Assignee have sought to design seal elements which provide for increased life once a crack begins in the seal element and to provide a means for slowing crack growth at different transitions in seal elements to ensure that regular inspections will discover a crack in the seal element prior to such a catastrophic failure even if one inspection misses the formation of the crack.